1. Field of the Invention
The present invention relates to a charge coupled device (CCD) type solid state image pickup device, and more particularly, to a method for manufacturing such a device having at least one vertical transfer portion and a horizontal transfer portion.
2. Description of the Related Art
A prior art CCD type solid state image pickup device includes a plurality of photo/electro conversion portions, a plurality of vertical transfer portions connected to the photo/electro conversion portions, a horizontal transfer portion connected to the vertical transfer portions, and an output portion. Also, there are two kinds of CCD's: a surface channel CCD(SCCD) where a signal charge is transferred along a surface of a semiconductor substrate, and a buried channel CCD(BCCD) where a signal charge is transferred within a semiconductor substrate.
An example of a SCCD is formed by an N-type semiconductor substrate, a P-type inpurity doped region formed within the semiconductor substrate, and a silicon dioxide layer formed on the P-type impurity region, thereby transferring a signal charge along a surface between the silicon dioxide layer and the P-type impurity doped region. On the other hand, an example of a BCCD is formed by an N-type semiconductor substrate, a P-type inpurity doped region formed within the semiconductor substrate, a N-type impurity doped region formed within the P-type impurity region and a silicon dioxide layer formed on the N-type impurity doped region, thereby transferring a signal charge along a depletion region between the N-type impurity doped region and the P-type impurity doped region.
Incidentally, in a prior art BCCD type solid state image pickup device for a high definition television (HDTV) system and a movie system, as the number of elements has been remarkably increased and the size of the device has been remarkably reduced, the size of the vertical transfer portions has been reduced. As a result, in the vertical transfer portions, a junction between the N-type impurity doped region and the P-type impurity doped is more shallow, to thereby enhance the ability to transfer charges per unit area. On the other hand, in the horizontal transfer portion, a junction between the N-type impurity doped region and the P-type impurity doped region is deepened to enhance the fringe electric field to thereby compensate for the efficiency of transfer of charges deteriorated by the enhanced ability of transfer of charges of the vertical transfer portions. That is, the impurity doped region for the vertical transfer portion has a different concentration distribution from the corresponding impurity doped region for the horizontal transfer portion. The same is true for the SCCD.
In a prior art method for manufacturing a CCD type solid state image pickup device having two impurity doped regions for a vertical transfer portion and a horizontal transfer portion, these regions having different concentration distributions from each other, the two impurity doped regions are manufactured independently, which will be later explained in detail. As a result, if the two impurity doped regions are largely superposed onto each other, a potential barrier may occur under a charge transfer electrode. Or if the two impurity doped regions are slightly superposed onto each other or are separated from each other, a deep potential well may occur under the charge transfer electrode. Both cases prevent a smooth transfer of charges from the vertical portion to the horizontal portion.